As someone who changes things in the system for fun, I can tell you that's not correct. I've fixed computers by working in the BIOS, but I have yet to ever mess anything up. Now, Apple just doesn't want people to run their OS on computers that aren't sold by them because, well, it's not sold by them. Beyond them not trying to make mac OSX specifically work with all hardware, there's nothing actually stopping you from doing it. Installing an operating system on a computer doesn't void any warranty, and in the very unlikely case that it does then he'd have already broken it when he installed linux. And if he can install linux, he should be able to install OSX.
I can also site many cases where users of a hackintosh crashed their system. But that's besides the point. It's illegal and yes, it will void the warranty because running a system it isn't meant to run counts as tampering. Dozens of computer techs will tell you that installing a foreign system, such as one that isn't meant to run on that machine by default voids the warranty. It's more of a legal matter and usually only applies with OSX, which cannot be installed on a PC legitimately or safely.
Crashing, in this case, is not harmful. Illegal, sure, but that's just piracy (or as Johnny said he has a mac OSX install disc, so possibly not even), and there is no warrantly loss for running a new OS on a computer, computers aremeant to run OSes, and that counts mac on non-mac computers too. That's just false, coming from a computer hobbyist, installing pirated software, while illegal, voids no warranty, and is easy to remove without a trace anyway (so no one would even have to know, let alone void your warranty for it). It is very safe.
I don't know, maybe I'm just old fashioned, but I think it's just best to go with the software it was meant to run and not to risk the system's integrity on something that can blow up in your face. I used to be a certified Apple repair tech and I can tell you it will void the warranty. It violates the DMCA as stated by the courts and if it is installed on an unauthorized machine such as Dell, Linux or anything that isn't Apple, they can refuse your warranty by all rights and most repair techs and shops will. You seem to know a lot, as do I, though I'm a little more based in hardware, not so much software, though I do know a good bit about it, save for coding, I never really had the time or ambition to learn it in high school and I should have. As far as it goes, you should be careful what you install on certain machines, it could crash the system and render it unbootable. I've seen a few cases in which exactly that happened because the user wasn't sure of what they were installing. My basic rule of thumb is if it wasn't meant to be installed on a certain OS, then it should be left at that. But that's just me, Idk.
The thing is, you're not risking the system, the worst an OS can do is not work. Void the warranty on the OS, maybe, but the system should still have the warranty, after all they're made to have new OSes introduced. As well, it's very possible to wipe the hard drive before going to a repair shop (if that itself doesn't solve the problem). If it becomes unbootable, it's just a matter of installing a new OS on it, in this day and age you'll probably have lots of friends who have extra computers if you don't have more than one yourself. My big rule of thumb is if you think it won't mess your computer up, you should do it. Maybe I've just gotten lucky with this, or know more than I think, but I've never run into a single issue yet.
It could be luck or a strong knowledge of coding, really but not everyone is as adept at that type of thing. Well, Windows machines are obligated by law to void the machine's warranty if they find OSX86 installed because of the legal ramifications with Apple. Dell, HP and all of the other PC companies are unwilling to risk a lawsuit and will most likely refuse your machine's warranty, unless like you said, you empty the hard drive. Other than that, it will void any warranty because it's just like opening the case to computer manufacturers. You open the case, you void the warranty, you install software it's not authorized for, you void the warranty. See, the difference between Apple and Microsoft is Microsoft sells the license to their OS, enabling it to be booted on any machine legally. Apple on the other hand, does not allow such distribution and limits the license to Apple products only. In all actuality, it's even illegal for Apple to factory install Windows, due to copyright protection. That is why the user must buy their own copy of Windows and install it themselves, via Boot Camp. Apple is permitted to make the application that runs the software but they are not permitted to factory install Windows. Apple does not permit any emulators, Hack versions of their software or applications to run their software on any other machine than their own. That's the difference between the two companies.